This invention relates to a polymeric material which is a conductive blend of at least two polymers and contains at least one finely divided conductive material. The invention also relates to methods for preparing the conductive blends of polymers, and their use.
Polymers in general are insulating materials. For certain applications it is desirable for a polymeric material to have some degree of electrical conductivity, for example in "ESD" or electrostatic dissipative applications such as anti-static packaging, housing for electronic equipment, containers and pipelines for flammable liquids and gases, or in charge-transporting components for electrographic imaging equipment.
The addition of finely divided conductive material such as a conductive carbon black is often used for achieving the desired conductivity in a polymer or polymer blend. The conductive carbon black is dispersed in the insulating polymer matrix. As the amount of dispersed particles of carbon black is increased and reaches the "percolation threshold" concentration, the particles come sufficiently into contact with each other so that a marked increase in conductivity is observed for the carbon black-loaded polymer. The desired conductivity is obtained by controlling the loading of the conductive particles such as carbon black. However, as the concentration of carbon black increases, the mechanical properties of the composite tend to deteriorate. The toughness and flexibility of the composite decrease, and an article formed from the carbon filled material is undesirably brittle.
In order to recover good impact strength and flexibility in thermoplastic polymers containing carbon black, a common method incorporates impact modifiers such as rubber particles, core/shell acrylic copolymers, thermoplastic elastomers, or other reinforcing agents into the thermoplastic composition. These additives increase process complexity and generally cause other side effects such as rheological modification or dispersion problems.
Another important detrimental effect of the presence of carbon black in plastics is the reduction of the melt fluidity of the thermoplastic polymers, which affects the ease of processability at the transformer level (at the injection molder, extruder, blow molder, thermoforming, etc.). A melt viscosity which is too high can lead to a reduction of output rates, higher energy consumption, increases in melt pressure and melt temperature, mold filling problems, and polymer degradation.
In view of the above mentioned detrimental effects of the incorporation of carbon black, it is desirable to reduce the amount of carbon black in the polymer composition to improves its global product property profile. For that reason, the carbon blacks used are generally superconductive ones such as KETJENBLACK EC 600 JD.TM. (AKZO) or PRINTEX XE2.TM. (DEGUSSA) in order to obtain electrical percolation at the minimum carbon black loading. It is, however, generally difficult, if not impossible, to obtain electrical conductivity for compositions containing, for example, less than 5% KETJENBLACK EC 600 JD.TM., which is a highly conductive carbon black.
Furthermore, although superconductive carbon blacks are generally preferred to others due to the lower loading necessary to obtain a percolation path, they present the worst structural characteristics that can be envisioned with regards to the problems described above. They are characterized by a high degree of structure (high DBPA) and small primary particles (high surface area). As a consequence, first, it is generally difficult to obtain good dispersion of these carbon blacks in the composition (dispersion being difficult for carbon black of small primary particle size), which results in deficiencies in mechanical properties, and, second, the viscosity of the obtained composition is high because of the combination of a high degree of structure and a small primary particle size. Consequently, even by using superconductive carbon black, it is not possible to reduce the level of carbon black sufficiently to overcome the problems described above.
More recently, another approach in reducing the carbon black loading necessary for imparting conductivity to a polymer has been investigated with specific blends of immiscible polymers which form two co-continuous phases (i.e., two simultaneously and separately continuous phases) in which the carbon black is localized selectively in a continuous polymeric phase, or at the continuous interface between the two co-continuous polymeric phases. Investigations of the following polymer systems had thus been reported:
HDPE/PS PA1 PS/PMMA PA1 PS/Rubbers (EPR, EPDM, polybutadiene, polyisoprene) PA1 HDPE/ultrahigh molecular weight PE PA1 PP/polyamide PA1 PS/polyisoprene PA1 PP/polycarbonate,
wherein PS = polystyrene PMMA = polymethyl methacrylate EPR = ethylene propylene rubber EPDM = ethylene propylene diene rubber HDPE = high density polyethylene PP = polypropylene.
In the above systems, under certain conditions the carbon black was found to be selectively localized in one of the continuous polymeric phases, or at the continuous interface between the co-continuous polymeric phases. As a result of the selective localization, conductivity was attained with a lower carbon black load. In particular, when the carbon black was localized at the continuous interface between the two co-continuous polymeric phases, an even higher conductivity was obtained (i.e., the percolation threshold concentration was greatly reduced) than when the carbon black was localized in a continuous polymer phase. However, the co-continuity of the blend which was required for obtaining this reduction of the percolation threshold concentration for electrical conductivity was not always achieved with all the reported blends.
Although the mechanical properties of the above co-continuous carbon black loaded-polymer blends are less impaired because the carbon black loading is less than in a single phase polymer matrix, the blending of two such polymers which are not fully miscible and not fully compatible nevertheless results in poorer mechanical properties of the blends, as compared to the mechanical properties of the polymer component in the blend which has the most desirable mechanical properties when used by itself (i.e., as a single phase, one-polymer composition). The reason is that there is poor interfacial adhesion at the phase boundary between the two polymers, which presents weak points at which fractures can find easy propagation paths.